The invention relates to an apparatus for the reactive coating of a substrate with an electrically insulating material, for example silicon dioxide (SiO.sub.2). It comprises an A.C. power supply which is connected to an electrode disposed in an evacuable coating chamber which in turn is electrically connected to targets to be sputtered. The sputtered particles thereof are deposited on the substrate while a process gas and a reactive gas are supplied to the coating chamber.
The problem in known processes for coating substrates by means of cathode sputtering using materials with a high affinity for the reactive gas is that aside from the substrate, parts of the apparatus like the inside wall of the process chamber or parts of diaphragms or the target surface are coated with materials that are not electrically conductive or only poorly conductive. This frequently entails an alteration of the process parameters and, particularly, electrical arcings which cause not only a frequent interruption of the process but also require a frequent cleaning or replacing of parts of the apparatus.
A sputtering apparatus (U.S. Pat. No. 3,860,507) is known which operates on radio frequency, preferably at 13.56 MHz, where two diametrically opposed targets are provided in the process chamber. These targets are connected to the two outputs of the secondary coil of an A.C. transformer via electrodes. The secondary coil has a central tapping which is electrically connected to the wall of the process chamber in such a manner that a glow discharge forms between the two targets.
Further, an apparatus is known (DE-OS 38 02 852) for coating a substrate with a material obtained from a plasma where the substrate is located between a first and a second electrode. The first electrode is connected to the first terminal of an A.C. power supply, and the second electrode to the second terminal of the A.C. power supply. In this case, the source of the alternating current is configured as a stray field transformer to which power is supplied from a shielded arc welding unit or a similarly controlled A.C. power supply. Moreover, the two electrodes can optionally be connected to a D.C. power supply.
Finally, a sputtering apparatus is known (DD 252 205 A1) which comprises a magnet system and at least two electrodes disposed thereabove which are made of the material to be sputtered and switched such that they alternatingly function as cathode and anode of a gas discharge. The electrodes are connected to a sinusoidal alternating voltage of preferably 50 Hz. Each electrode is associated with an independent magnet system where one pole of the one magnet system is also the pole of an adjacent magnet system. The electrodes are disposed in one plane.